Ok , but on what basis do you believe in it?
I did not find a single Bible text there that says that people suffer some form of torment/suffering after death to make them ready to enter heaven.
ok so then believers in Catherine would have a possible reason for considering Purgatory
from: 
CATHOLIC ENCYCLOPEDIA: Belief
Purgatorial fire
At the Council of Florence, Bessarion argued against the existence of real purgatorial fire,
and the Greeks were assured that the Roman Church had never issued any dogmatic decree on this subject.
In the West the belief in the existence of real fire is common.
Augustine (Enarration on Psalm 37, no. 3) speaks of 
the pain which purgatorial fire causes as more severe than anything
a man can suffer in this life, "gravior erit ignis quam quidquid potest homo pati in hac vita" (P.L., col. 397).
Gregory the Great speaks of those who after this life "will expiate their faults by purgatorial flames,"
and he adds "that the pain be more intolerable than any one can suffer in this life" (Ps. 3 poenit., n. 1).
Following in the footsteps of Gregory, St. Thomas teaches (IV, dist. xxi, q. i, a.1) that 
besides
 the separation of the soul from the sight of God, there is
 the other punishment from fire.
"Una poena damni, in quantum scilicet retardantur a divina visione; alia sensus secundum quod ab igne punientur",
and St. Bonaventure not only agrees with St. Thomas but adds (IV, dist. xx, p.1, a.1, q. ii)
that this punishment 
by fire is more severe than any punishment which comes to men in this life;
 "Gravior est omni temporali poena. quam modo sustinet anima carni conjuncta".
How this fire affects the souls of the departed the Doctors do not know, and in such matters it is well to
heed the warning of the Council of Trent when it commands the bishops
"to exclude from their preaching difficult and subtle questions which tend not to edification',
and from the discussion of which there is no increase either in piety or devotion" (Sess. XXV, "De Purgatorio").